In downhole operations, such as in bores for reservoirs (e.g. oil and gas reservoirs), downhole tools are often required to be rotated, such as for drilling the bore.
In some operations rotation is temporarily or selectively transmitted downhole. For example, in directional or controlled trajectory drilling, a steering portion of the downhole tool may be rotated only when the direction of drilling is changed; whilst the drill bit may be rotated more of the time.
In directional drilling, the vertical inclination and azimuth of a drilled bore may be controlled such that the bore may extend from the surface to a target area which is not vertically aligned with the point on the surface where drilling commences. This permits a wide area to be accessed from a single drilling location and is therefore particularly useful in offshore drilling operations.
Applicant's GB 2,343,470 and U.S. patent application Ser. No. 09/435,453, and also WO97\47848 and U.S. patent application Ser. No. 09/202,342 and U.S. patent application Ser. No. 10/470,031, the disclosures of which are incorporated herein by reference, describe arrangements including non-rotating offset masses to provide a desired offset of the drill string in the bore.
In some downhole operations there can be changes in the transmission of rotational drive that result in a driven component being inadvertently coupled or decoupled; or coupled or decoupled under undesirable conditions. For example, where a drive coupling is controlled by a fluid pressure or a fluid pressure differential, the driven component may be inadvertently coupled by an unplanned change in fluid pressure (e.g. if a pump fails). Undesirably transmitting drive to components can potentially damage the driven components or other parts of the downhole tool or associated equipment; or cause delay or impede operations.
It is among the objectives of at least one embodiment of at least one aspect of the present invention to seek to obviate or at least mitigate one or more problems and/or disadvantages of the prior art.